This invention relates to a process for recovering valuable metals from spent catalysts and is especially directed to recovering cobalt from spent catalysts containing cobalt components intimately admixed or supported on aluminum-containing refractory oxides. In particular, the extraction process of the present invention is applicable to spent hydroprocessing catalysts, that is, catalysts utilized in particulate form and under conditions of elevated temperature and pressure and in the presence of hydrogen to convert a hydrocarbon liquid to more valuable products.
Typically, cobalt-containing hydroprocessing catalysts are employed in the petroleum industry for hydrogenation, hydrotreating, hydrodesulfurization, hydrodenitrogenation, hydrocracking, and hydrofinishing. Usually, cobalt-containing catalysts used in the foregoing processes also contain molybdenum components, and both the cobalt and the molybdenum components are largely present as sulfides intimately composited with a refractory oxide such as alumina. The proportion of cobalt components (as CoO) in such catalysts is between 2 and 20 weight percent and the proportion of molybdenum components (as MoO.sub.3) is usually between 8 and 35 weight percent.
It can be seen that, when spent, the typical hydroprocessing catalyst provides a relatively rich source of cobalt or cobalt and molybdenum. At present-day prices of $20.00 per pound for cobalt and $7.00 per pound for molybdenum, there is a strong incentive to recover as much of these metals as possible from the spent catalyst particles.
One method by which the active metal in a catalyst may be recovered is by total digestion, i.e., by dissolving the catalyst in hot, concentrated sulfuric acid. Although this method has the obvious advantage of recovering 100% of the available metals, it also has disadvantages. The amount of acid necessary for total digestion is prohibitive from an economic standpoint, and the resulting solution contains far more dissolved aluminum than active metals, making separation of the valuable active metals from the relatively inexpensive aluminum a difficult problem.
Accordingly, it is an object of the invention to provide a process for recovering cobalt or cobalt and molybdenum in substantial proportions from a catalyst also containing an aluminum-containing refractory oxide. It is a further object to recover such metals by liquid extraction, and to extract far less aluminum than active metals, thereby simplifying recovery of the active metals from the leachate. These and other objects and advantages will be apparent from the following description of the invention.